1. Field of the Invention
The present invention relates to a control apparatus for a continuously variable transmission of a vehicle which can continuously vary the output of an internal combustion engine equipped in the vehicle.
2. Description of the Prior Art
A conventional control apparatus for a continuously variable transmission of the type mentioned above is known, for example, from Japanese Patent Publication No. 7-102791 and Laid-open Japanese Patent Application No. 2001-355718. The control apparatuses described in these patent documents provide a continuously variable transmission mode and a stepped transmission mode for the continuously variable transmission. In these control apparatuses, the two transmission modes are automatically switched in accordance with a throttle opening indicative of the degree of acceleration required by the operator. Specifically, the continuously variable transmission is set to the continuously variable transmission mode when the throttle opening is smaller than a predetermined value, and is switched from the continuously variable transmission mode to the stepped transmission mode when the operator largely treads down on an accelerator pedal to cause the throttle opening to increase to the predetermined value or more. Then, in the continuously variable transmission mode, a target transmission ratio is continuously set in accordance with the vehicle speed and the like, and the transmission ratio of the continuously variable transmission is continuously controlled to reach the target transmission ratio. In the stepped transmission mode, on the other hand, the continuously variable transmission is controlled by the control apparatus of Japanese Patent Publication No. 7-102791 or Laid-open Japanese Patent Application No. 2001-355718 in the following manner.
First, in the controller of Japanese Patent Publication No. 7-102791, one of a plurality of previously set transmission stages is selected in accordance with the vehicle speed, throttle opening, and the like. Then, the transmission ratio of the continuously variable transmission is controlled on a stage-by-stage basis to reach a target transmission ratio which is a predetermined transmission ratio corresponding to the selected transmission stage. In the control apparatus of Laid-open Japanese Patent Application No. 2001-355718, a target transmission ratio is the sum of a basic transmission ratio equal to the transmission ratio of the continuously variable transmission immediately before the transmission is switched from the variable continuous transmission mode to the stepped transmission mode, and an additional transmission ratio, and the transmission ratio of the continuously variable transmission is controlled to reach the target transmission ratio. The additional transmission ratio, which is set in accordance with the throttle opening and vehicle speed, is set to a predetermined lower limit value when the vehicle speed exceeds a predetermined value, resulting in the target transmission ratio fixed to a constant value. This control apparatus also controls the transmission ratio of the continuously variable transmission such that the rotational speed of the engine does not exceed a predetermined upper limit rotational speed in order to prevent excessive rotation of the engine.
The control apparatuses of Japanese Patent Publication No. 7-102791 and Laid-open Japanese Patent Application No. 2001-355718 suffer from the following problems. In the control apparatus of Japanese Patent Publication No. 7-102791, since the target transmission ratio in the stepped transmission mode is set to a predetermined transmission ratio for each transmission stage, the operator tends to have a discomfort caused by discontinuous switching of the transmission ratio when the transmission is switched from the continuously variable transmission mode to the stepped transmission mode. Also, the control apparatus sets a plurality of transmission stages based on the following map. The map, which takes the vehicle speed on the horizontal axis and the throttle opening on the vertical axis, respectively, sets a boundary of switching between adjacent transmission stages to substantially increase as the vehicle speed is higher, with its slope being smaller toward a higher speed at which a larger transmission stage is set. Thus, when the transmission is switched from one stage to another in the stepped transmission mode using such a map, the boundary is often passed, particularly when moderate acceleration is required, causing frequent switching of the transmission and a resulting degradation in drivability.
In the control apparatus of Laid-open Japanese Patent Application No. 2001-355718, on the other hand, the target transmission ratio is fixed to a constant value when the vehicle speed exceeds a predetermined value, and the transmission ratio of the continuously variable transmission is controlled to prevent the engine rotational speed from exceeding a predetermined upper limit rotational speed, as described above. Therefore, even if moderate acceleration is required, the engine rotational speed continues to increase unless the engine rotational speed reaches the upper limit rotational speed, causing an associated discomfort to the driver. In addition, when a highly accelerating condition continues, the engine rotational speed becomes constant as it reaches the upper limit rotational speed, again causing a discomfort to be given to the driver. Thus, this control apparatus disadvantageously fails to appropriately respond to a request for acceleration from the driver.